This seems to me the big benefit here -- enough penetration of home backup batteries means public power generation doesn't have to be built up to provide massive peak surges. Further, trickle charging the backup batteries during the evenings/nighttime and allowing them to meet some needs during the day also means a higher net usage of generated power, so perhaps levels of generation could go down altogether.

There's also then the capitalistic angle -- if you want more peak power at your house, you can buy more batteries in lieu of (or in concert with) installing secondary power systems.

But reducing gross energy production due to higher net utilization would be a really cool thing, so long as the total cost to the macro system (considering full life of the battery vs. load taken off power plants) is a net positive.